Bicycles, or bikes, and other velocipedes come in a variety of shapes and sizes and are designed and used for a variety of purposes. For example, velocipedes may be used for leisure activity, for exercise, for touring, for entertainment, for sport, for business, for cargo hauling, for commuting, for general transportation, etc. Typical bicycles are often classified as one or more of BMX, road, cyclocross, racing, track, touring, utility, commuter, mountain, off-road, downhill, time-trial, triathlon, cruiser, etc.; however, such classifications, or types, of bicycles are certainly not exhaustive and a given bicycle may be used for a variety of purposes regardless of a so-called classification or type for which it is designated or designed to be used.
FIG. 1 illustrates a typical, standard bicycle frame 10, which also may be referred to as a diamond frame due to the side profile of such frames. As indicated in FIG. 1, a standard diamond frame includes a top tube 12, seat tube 14, and a down tube 16. The top tube, seat tube, and down tube are often described as forming a front, or main, triangle 18; however, as seen in at least the illustrated example, these three frame structures may not form a true triangle. For example, a standard diamond frame also typically includes a head tube 20, which in the illustrated example generally forms a quadrilateral together with the top tube, the seat tube, and the down tube. The head tube defines a connection and pivot point (and/or axis of rotation) for a corresponding front fork, to which a bicycle's handlebar and front wheel are coupled. A diamond frame typically also includes a pair of seat stays 22 and a pair of chain stays 24, both terminating at a pair of rear drop-outs 26 at the lower ends thereof. The seat stays typically are coupled directly to the seat tube 14 at the upper ends thereof, as seen in FIG. 1. The seat stays, together with the chain stays and the seat tube form what is often described as a rear triangle 28, again, not necessarily forming a true triangle. The drop-outs are structures that are configured to receive an axle of a corresponding rear wheel of a bicycle to rotationally couple the rear wheel to the frame. A bottom bracket 30 is positioned at the junction of the down tube, the seat tube, and the chain stays, and is where a corresponding crank set of a bicycle is attached. All of the top tube, the seat tube, down tube, seat stays, and chain stays of a typical diamond frame are linear, or at least predominantly linear.
In a traditional diamond frame, such as in the example illustrated, the top tube generally extends at least approximately parallel to the ground surface, when the frame is part of a complete bicycle with front and rear wheels. This frame geometry may be referred to as a traditional geometry. Somewhat recently for road bike frames, a so-called compact geometry has become popular. In a compact geometry bicycle frame, the top tube slopes downward from the head tube to the seat tube, and generally the seat stays connect to the seat tube at approximately the same height as the top tube. Various other non-traditional, or non-standard, frame designs have been used throughout the history of the bicycle.
The aforementioned structural components of bicycle frames are referred to as tubes because historically, these structures were in fact constructed of cylindrical tubes. For example, steel tubing has long been used to construct bicycle frames. More recently aluminum, titanium, and other metal alloys have been used to construct frames, with such materials not necessarily being formed in cylindrical tubes. For example, ovular tubes, or even rectangular tubes are sometimes used. Various other materials also are used to construct frames, such as wood and bamboo.
Somewhat recently, carbon fiber has been used to construct bicycle frames, and in particular high performance road bicycle frames, including frames constructed completely of carbon fiber, as well as composite frames with only portions constructed of carbon fiber. Composite materials that include boron fibers and/or Kevlar fibers also have been used to construct bicycle frames. Such composite materials lend themselves to being formed into a variety of shapes and constructions for bicycle frames. Therefore, frames constructed of such composite materials do not necessarily include linear sections of tubing, and a variety of frame geometries have been employed utilizing composite materials.
Some bicycles may be described as having active suspension systems, such as including pivot points between frame members, shock absorbers, springs, etc. Mountain bikes and downhill bikes are examples of bicycles that may include active suspension systems. When including active suspension systems, such bicycle frames may resemble, or include aspects of, a typical diamond frame with a top tube, a down tube, and a seat tube, while others may not resemble typical diamond frames and may not include one or more of a top tube, a down tube, a seat tube, and seat stays.
Bicycles without active suspension systems may be described as having passive suspension systems, in so far as the various frame members are rigidly (and/or directly or permanently) connected to each other and do not include pivot points, shock absorbers, springs, etc. Performance bicycle frames (e.g., road frames) with passive suspension systems are sometimes described in terms of stiffness to weight (STW) ratios. Various stiffnesses of frames may be measured, including the vertical stiffness, or compliance, of a frame, the lateral (or torsional) stiffness of a frame, as well as the stiffness of individual frame members, such as the bottom bracket of a frame. For performance bicycle frames, manufacturers attempt to optimize these various STW ratios, so that the frame is lightweight, yet highly stiff in certain directions, for example, to ensure that the rider's pedal stroke is efficiently transferring power to the bicycle's wheels and ultimately to the ground.
With reference to FIG. 2, a schematic illustration of a suitable (but not exclusive) test for measuring the lateral (or torsional) stiffness of a frame is provided. As illustrated, the frame is positioned on its side (i.e., with the head tube in a horizontal orientation), and the rear drop-outs are immobilized. A bar, rod, or similar stiff shaft (with an illustrative non-exclusive example being a two-meter steel bar) is positioned through and centered in the head tube, and a predetermined force (such as a one-Newton force) is applied to one end of the shaft. The predetermined force also may be applied by coupling and suspending therefrom a preselected mass 32 to the shaft to thereby apply a known force at a known distance away from the top tube. The deflection 33 of the opposite end of the shaft is measured to provide the lateral stiffness of the frame. This stiffness also may be presented relative to the weight of the frame and may be expressed in terms of a STW ratio.
With reference to FIG. 3, a schematic illustration of a suitable (but not exclusive) test for measuring the vertical stiffness, or compliance, of a frame is provided. The initial set-up for the test may correspond to section 4.8.4.3 of the European Standard for racing bicycle safety (EN 14781 November 2005). More specifically, a frame together with a front fork is positioned in its normal position of use, with the front and rear axles being horizontal with respect to each other, with the rear axle being able to pivot, and with the front fork supported on a flat steel anvil. A mass 34 of 70 kg is positioned on a seat post so that the distance 35 along the seat-post from its center of gravity to the seat post's insertion point in the frame is 75 mm. The deflection of the mass in the vertical direction is measured and may then be expressed in terms of distance per unit force (e.g., mm/kN). With typical diamond frames, the ratio of the vertical displacement of the bottom bracket to the vertical displacement of the mass is close to one. The displacement of the bottom bracket corresponds to the stiffness of the frame, and thus affects the performance, or efficiency, of a performance bicycle. For example, the greater the displacement of the bottom bracket, the more the forces of a rider's pedal stroke are absorbed by the frame as opposed to being transferred to the bicycle's wheels. Conversely, the lesser the displacement of the bottom bracket, the less the forces of a rider's pedal stroke are absorbed by the frame, and the more the forces are efficiently transferred to the bicycle's wheels and ultimately to the ground.